The Sydney Basin is a major structural basin containing a thick Permian-Triassic (290 Ma - 200 Ma (million years old)) sedimentary sequence that is part of the much larger Sydney-Gunnedah-Bowen Basin. This extends all the way from Durras Lake (near Batemans Bay), in the south of New South Wales, north to Bowen (just south of Townsville) in Queensland, a distance of several thousand kilometres. The Sydney Basin is economically important as it contains all the known large coal-fields in New South Wales and Queensland.

Sydney Harbour is made up of mostly sandstone and shale formed during the Triassic period (about 220 million years ago). These formations were later raised to their present heights by earth movements, starting in the Jurassic period, (200 million years ago). During this time great cracks formed and molten lava rose up through the rocks to form volcanic vents, these then cooled and hardened to form dykes of basalt. Remains of basalt dykes can still be found around Bradleys Head, North Head and Nielsen Park.

Perhaps the most important geological deposits are those that we use for building purposes. These come from all geological environments. The most important economic factor for building materials is that the material has to be close to where it is going to be used, as the highest cost is in its transportation. Building materials are by far the lowest cost geological materials and their value is usually in the order of only a few dollars per ton.